Level shifters, sometimes called level translators, interface between lower voltage circuitry and higher voltage circuitry (e.g., in logic systems and/or power systems), such as by safely translating signals between the higher voltage circuitry and the lower voltage circuitry. For example, a level shifter may isolate and protect the lower voltage circuitry against higher voltages (from the higher voltage circuitry) that could otherwise damage the lower voltage circuitry.
Example applications for level shifters include automotive systems. In an automotive system, the battery voltage is a DC voltage that can vary from as low as 3 volts to as high as 40 volts, depending on alternator or generator operation, ambient or engine temperature, and loading, while a nominal battery voltage is about 12 volts. Automotive systems include power field effect transistors (“power FETs”), but these power FETs are not rated for a full range of battery voltages that might occur. Instead, these power FETs may have an example safe voltage range of 3.3 to 5 volts for a gate-to-source voltage (Vgs), and they may have low on-resistance and low capacitance. To avoid permanently damaging these power FETs, the automotive system must ensure that gate voltages are within the safe voltage range.
Additional applications for level shifters include switching power converters that can require operating frequencies as high as 10 MHz on an input terminal. In a DC-DC converter application, the level shifter speed (at least in part) restricts a minimum duty cycle that the switching converter can use and thus limits a level of voltage conversion that can be achieved. For example, in some converters, the level shifters must have fast rise and fall times (Tr, Tf) to achieve the required operating frequencies, such as less than 5 nanoseconds.
Some level shifters include drain extended metal oxide semiconductor (DEMOS) transistors. A DEMOS transistor has an extended and lightly doped drain area that reduces an electric field under its gate. This reduced electric field increases a drain-to-source (VDS) breakdown voltage (in comparison to other MOS transistors), so a DEMOS transistor is useful to interface between two power domains that have different voltage operating levels, because the DEMOS transistor is suitable for coupling to different levels of voltages. In comparison to other MOS transistor types, DEMOS transistors have more parasitic capacitance, and occupy more silicon area.
Also, level shifters with DEMOS transistors devices can exhibit asymmetric signal translation, high power usage and low bandwidth.